Rockey Cheng,
There seems to be need for more flexibility between the pipe rack and the buried piping, but the use of an expansion joint must be carefully considered. A simple expansion joint without any tie rods may have enough axial movement for the settlement, however the cross section area for 48" diameter may result in very high pressure thrust load. The thrust load for 50 lb/in cooling water could result in almost 86,750 lbs uplift to the pipe on rack. The tie rods are usually installed on an expansion joint to control the pressure thrust, but then there is not the axial movement needed.
The buried pipe might need to have a thrust anchor at the elbow directly below the pipe rack. The civil engineer probably would want to include the underground thrust anchor as part of the foundation footing of the pipe rack. If the thrust anchor and the foundation footing are thus connected, then differential settlement would not occur.